bmi Unit 2_opt.pdf

8/11/2019 bmi Unit 2_opt.pdf

1/56

www.eeecube.com

www.eeecube.com
http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/


2/56

An Overview . . .

Basic components of a biomedical system

Electrodes

Amplifiers

ECG

EEG

EMG

ERG

www.eeecube.com

www.eeecube.com


3/56

o

oo

o

oBiological

SystemTransducer Signal

Processor

FeedbackTransform

SurgicalTool

Measurement

Monitoring

Diagnosis

Surgery

Therapy

Block Diagram of a Generalised Bio Medical Instrument System

www.eeecube.com

www.eeecube.com


4/56

Each switch position connects an instrument for measurement, for

monitoring, for diagnosis, for therapy or for surgery with the signal

processor

Transducer converts one form of energy or signal to another, its

output is always an electrical signal

Signal Processor amplifies, modifies or changes the electrical

output of the transducer

In the case of therapy, the signal is fed back to the biological system

through the feedback transform

In the case of surgery, a surgical tool is in contact with the biological

system

www.eeecube.com

www.eeecube.com


5/56

Electrodes

Employed to pick up the electrical signals of the body Hall cell potential voltage developed at an electrode electrolyte interface

Perfectly polarised electrodes no net transfer of charge occurs across the

metal electrolyte interface

Perfectly non polarisable electrodes unhindered exchange of charge is

possible across the metal electrolyte interface

Electrolyte paste - used to reduce the contact impedance

Types

Microelectrodes

Depth and Needle Electrodes

Surface Electrodes

www.eeecube.com

www.eeecube.com


6/56

Microelectrodes

Have smaller diameter

No damage to the cells during insertion

To measure the potential they, located within the cell

Reference electrode is outside the cell

Types

Metallic

Non Metallic (Micropipet)

www.eeecube.com

www.eeecube.com


7/56

RFA

EA

EC( t)RIN REX

CWA

RWA

A

B

RFB

RWB

CWB

RA

RB

RS

CD

o

o

Metal Microelectrode

EB

www.eeecube.com

www.eeecube.com


8/56

RA denotes the resistance of the connecting wire (negligible)

RS denotes the resistance of the shaft of the microelectrode (also negligible)

RFA, RWA and CWA constitute the impedance of the microelectrode tip

intracellular fluid interface

RIN is the resistance of the intracellular fluid

RB denotes the resistance of the connecting wire to the reference electrode

(negligible)

RFB, RWB and CWB constitute the impedance of the reference electrode tip

extra cellular fluid interface

REX is the resistance of the extra cellular fluid

CD is the distributed capacitance between the insulated shaft of the

microelectrode and the extra cellular fluid

The capacitance between the tip of the microelectrode and the intracellular

fluid is negligiblewww.eeecube.com

www.eeecube.com
http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/


9/56

o

o

EC

E = EA - EB

RFA

CWA RWA

CD

A

B

Equivalent Circuit

www.eeecube.com

www.eeecube.com


10/56

Since the area of the reference electrode is many times greater than

the metal electrodes tip whose area of cross section is very small, its

impedance is negligible

The impedance of the microelectrode tip is inversely proportional to

the area of the tip and frequency

When the electrode output is coupled with an amplifier, the lowfrequency components of the bioelectric potential will be attenuated

if the input impedance of the amplifier is not high

Thus when the input impedance of the amplifier is not high enough,

it behaves as a high pass filter

www.eeecube.com

www.eeecube.com
http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/


11/56

o

o

RTED

EC

RIN

REX

CD

EB

RFB

RWB CWB

EA

RFA

RWA CWA

RA

RB

A

B

Micropipet

www.eeecube.com

www.eeecube.com


12/56

RA denotes the resistance of the connecting wire

RFA, RWA and CWA constitute the impedance of the electrode

electrolyte interface in the stem of the micropipet

RT is the resistance of the electrolyte filling the tip of the micropipet

(very large)

RIN is the resistance of the intracellular fluid

RB denotes the resistance of the connecting wire to the reference

electrode

RFB, RWB and CWB constitute the impedance of the reference

electrode electrolyte interface

REX is the resistance of the extra cellular fluid

CD is the distributed capacitance between the fluid in the pipet and

the extra cellular fluidwww.eeecube.com

www.eeecube.com


13/56

o

o

EC

E

RT

CD

A

B

Equivalent Circuit

www.eeecube.com

www.eeecube.com


14/56

CD is the equivalent of distributed capacitances

When the micropipet is coupled with the amplifier terminals A and

B, the membrane potential EC is coupled with it via a high series

resistance RT and a moderate shunt capacitance CD along with

electrode potentials

The impedance of the electrode places a limit on the response time

of the circuit

It behaves as a low pass filter when the input impedance of the

amplifier is not high enough

www.eeecube.com

www.eeecube.com


15/56

o

o

o

o

Vo

V2

V1 .

.

e1

e2

Ri

Ri

Rf

Rf

Simple Differential Amplifier

www.eeecube.com

www.eeecube.com


16/56

Since the input current to an ideal amplifier is zero and by the principle

of superposition, the voltage existing at the input terminal 1 is,

e1 =Rf

Ri + Rf

V1 +Ri

Rf + Ri

Vo

Voltage at input terminal 2 is,

e2 =Rf

Rf + Ri

V2

Since the potential difference between the two input terminals of an

ideal amplifier is forced zero by the feedback through Rf, the voltage e1

must be equal to e2

www.eeecube.com

www.eeecube.com


17/56

Therefore,

Rf

Ri + Rf

V1 +Ri

Rf + Ri

Vo =Rf

Rf + Ri

V2

Ri

Vo =Rf (V2 - V1)(or)

Thus the circuit amplifies the difference of the input signals V1

and V2

There is no virtual ground at the input to the amplifier in this circuit

www.eeecube.com

www.eeecube.com


18/56

Modes of operation of the differential amplifier

Single ended mode either V1 (non inverting mode) or V2

(inverting) equal to zero

Differential mode the two input signals are equal but have

opposite polarity

Common mode the input signals are identical both in amplitude

and phase

www.eeecube.com

www.eeecube.com


19/56

Isolation Amplifier

Used to increase the input impedance of the monitoring system in

order to isolate the patient from the bio medical instrument

Used so that any electrical fault in signal conditioning or other

circuits does affect the patient

Some isolation amplifier circuits . . .

Darlington Pair

Bootstraping Circuit

www.eeecube.com

www.eeecube.com


20/56

o o

o

o

..

Q2

Q1

RB RL

- VBB - VCC

X

Y

Z

Zi

Darlington Pair

www.eeecube.com

www.eeecube.com


21/56


22/56

o

o

o

o

INPUT

OUTPUT

Q1

Q2

R1

R2R3

R4

C

Re Re

+ VBB


www.eeecube.com

www.eeecube.com


23/56


A feedback network is connected in between the emitter of Q2 and

the collector of Q1

The feedback voltage created b the bootstrap voltage dividing

network R1 and R2 is injected into the collector circuit of Q1

An increase in signal level at the input of the circuit causes an

increase in signal through the divider at the collector, changing the

transistors bias point

This further increases the input impedance

www.eeecube.com

www.eeecube.com


24/56

.

.

.

.

.

.RA

LA

LL

RL

ElectrosurgeryFilter

High Voltage andOver Voltage

Protection

Lead Selector

STD1mV

D.C.Amp

Guard

SynchronousModulator

Modulator

Driver

Transformer

SynchronousDemodulator

100 kHzOscillator

Rect.&

FilterFloatingCommon

+ V

- V

Isolated PowerTransformer

ECG Isolation Amplifier Circuit

www.eeecube.com

www.eeecube.com


25/56

The signals from the different leads are given to the low pass filter having a

cut off frequency of about 10 kHz

This is followed by high voltage and over voltage protection circuits

The signals are now into the lead selector switch which is used to derive the

required lead configurations

Its output is given to a d.c. amplifier ( also used for calibration purposes )

The primary of an isolated low capacitance power transformer is connectedwith the 100 kHz oscillator

The secondary of that transformer along with the rectifier and filter is used to

obtain isolated power supply of + 6V

The synchronous modulator modulates the ECG signal from the d.c. amplifier

Another transformer is used to deliver the output from the modulator to the

synchronous demodulator

Its output isinput to the power amplifierwww.eeecube.com

www.eeecube.com
http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/


26/56

o

oo

oo

Vout

V1

V2

R1

aR1

aR1

bR2

bR2

R2

R2

1

2

3

-

-

-

+

+

+

Vo

Vo

Medical Preamplifier Circuit

www.eeecube.com

www.eeecube.com


27/56

Amplifies the differential signal and rejects the common modesignal

Consists of three operational amplifiers

First two are working in non inverting mode, but their inverting terminals

are not grounded

The feedback loops are connected with the inverting terminals

The third acts as a differential amplifier

With this configuration we get

High Stability

High Fidelity

High CMRR

High input impedance

By means of virtual ground, the inverting terminal of op amp 1 is fed by

voltage V2 through R1 and the inverting terminal of op amp 2 is fed by

voltage V1

through R1www.eeecube.com

www.eeecube.com
http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/


28/56

Chopper Amplifier

Used to convert the d.c. or low frequency signal into a high

frequency signal

Then this modulated high frequency signal is amplified by

conventional a.c. amplifier

The amplified signal is demodulated and filtered to get amplified

d.c. or low frequency signal

Types

Mechanical Chopper Amplifier

Non Mechanical Chopper Amplifier

www.eeecube.com

www.eeecube.com


29/56

o

o

o

o

A

C D

S1

o

o

VoVi

P

Q

Chopper Amplifier Using a Mechanical Switch

M

N

www.eeecube.com

www.eeecube.com


30/56

The chopper S1 is an electromagnetically operated switch or relay

It connects alternatively the input terminal of the a.c. amplifier A to the

reference terminal Q, which is usually connected to ground When the amplifier input terminal is connected with Q, it is short circuited

and the input voltage is zero

When the chopper S1 is open, the amplifier receives the signal voltage from P

Therefore the input to the amplifier consists of an a.c. voltage varying from

zero to the value of the input voltage

A steady d.c. or slowly varying signal is chopped into a train of square wave

pulses having a frequency equal to the rate of the chopper

After amplification the chopped signal is rectified with a diode D

The rectified signal is then filtered and amplified d.c. is obtained at the

output terminals M and N

www.eeecube.com

www.eeecube.com
http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/


31/56

Photo Diodes

Low LevelDC Input

o o

AmplifiedDC Output

Oscillator

AC

Amp.

Modulator Demodulator

PC1

PC2PC3

PC4

Neon Bulb(1)

Neon Bulb(2)

Low Pass

Filter

Non Mechanical Photoconductive Chopper Amplifier

www.eeecube.com

www.eeecube.com


32/56

The photoconductors or photodiodes are used as non mechanical

choppers for modulation and demodulation

When there is no light on the photoconductor, its resistance is so many

mega ohms, it is in the reverse bias and no current is allowed to flow

through it

When there is incident light on the photoconductor its resistance is very

low, it is in the forward bias and current can easily flow through it

Thus it acts as a switch by means of incident light

An oscillator is present which drives two neon bulbs into illumination on

alternate half cycles of oscillation

The neon bulb (1) gives flash of light on photoconductors PC1 and PC2

which are respectively connected at the input and the output

The neon bulb (2) gives flash of light on photoconductors PC3 and PC4

Low level d.c. is present at the input

www.eeecube.com

www.eeecube.com


33/56

Whenever light falls on PC1 its resistance decreases and input capacitor

charges

When there is no light on PC1 and there is light on PC3, the input flows

through PC3

By the alternate incident light, a square wave is produced across the

capacitor

Its amplitude is proportional to the input and frequency is equal to thefrequency of the oscillator

An amplified square wave voltage is obtained at the output of the amplifier

The two photoconductors PC2 and PC4 in the amplifier output circuit

recover the d.c. signal by their demodulating action and the output

capacitor becomes charged to the peak of the output voltage

Then this d.c. voltage is passed through a low pass filter to remove any

ripples and finally amplified d.c. output is obtained

www.eeecube.com

www.eeecube.com


34/56

Electrocardiography

ECG deals with the study of the electrical activity of the heart muscles

Heart is divided into four chambers

The top two are atria and the lower two are ventricles

The right atrium receives blood from the veins and pumps it into the

right ventricle

The right ventricle pumps the blood into the lungs where it is purified

and oxygenated

The oxygen enriched blood enters the left atrium from which it is

pumped into the left ventricle

Then the left ventricle pumps the blood into arteries through aortic

valve for circulation throughout the bodywww.eeecube.com

www.eeecube.com


35/56

Each action potential in the heart originates at the Sino - Atrial (SA)

node which is situated in the wall of the right atrium and near the

entry of the vena cava

Also called as Cardiac Pacemaker and generates impulses at the

normal rate of the heart, about 70 beats per minute at rest

The action potential contracts the atrial muscle and the impulsespreads through the atrial wall during a period of about 0.04 second

to the Atrio Ventricular (AV) node

The AV node acts as a delay line to provide timing between the

action of the atria and the ventricles

Then a special conduction system carries the action potential to the

ventricular muscles

www.eeecube.com

www.eeecube.com


36/56

This system consists of a short common part (the bundle of His),

two bundle branches on each of the septum and fine Purkinje fibers

which arborize in the ventricular muscle

The atria and ventricles are functionally linked only by the AV node

and the conduction system

The AV delay is provided so that the atrial contraction can complete

the ventricular filling before the contraction of ventricles

www.eeecube.com

www.eeecube.com


37/56
http://www.eeecube.com/http://www.eeecube.com/


38/56

ECG Lead Configurations

Usually surface electrodes are used with jelly as electrolyte between skinand electrodes

The potentials generated in the heart are conducted to the body surface

The potential distribution changes I a regular an complex manner during

each cardiac cycle

So standardised electrode positions are chosen

Types of electrode systems

Bipolar Limb Leads (or) Standard Leads

Augmented Unipolar Limb Leads

Chest Leads (or) Precordial Leads

Frank Lead System (or) Corrected Orthogonal Leads

www.eeecube.com

www.eeecube.com


39/56

Bipolar Limb Leads

In standard leads, the potentials are tapped from four locations

Right Arm

Left Arm

Right Leg

Left Leg

The right leg electrode is used as ground reference electrode

www.eeecube.com

www.eeecube.com

b


40/56

. .

Vout Vout

+

-

+

-

Lead I Lead II

Standard Bipolar Limb Leads

www.eeecube.com

www.eeecube.com

b


41/56

.

Vout

+

-

Lead III

www.eeecube.com

www.eeecube.com

b


42/56

Lead I Position gives voltage drop from the left arm to the right arm

Lead II Position - gives voltage drop from the left leg to the right arm

Lead III Position gives voltage drop from the left leg to the left arm

www.eeecube.com

www.eeecube.com

b


43/56

-

+ +

+-

-

Lead I

Lead IIILead II

Right Arm Left Arm

Left Leg

The Einthoven Triangle

www.eeecube.com

www.eeecube.com

eeec be com


44/56

The closed path RA to LA to LL and back to RA is called the

Einthoven Triangle

According to Einthoven, in the frontal plane of the body the cardiac

electric field vector is a two dimensional one

The ECG measured from any one of the three limb leads is a time

variant single dimensional component of that vector

Along the sides of this triangle the three projections of ECG vector

are measured

Further the vector sum of the projection on all the three sides is

equal to zero

Thus, following Kirchoffs law, the R wave amplitude of Lead II is

equal to the sum of the R wave amplitudes of leads I and III

www.eeecube.com

www.eeecube.com

www eeecube com
http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/


45/56

. .

Vout Vout

+

-

+

-

Lead aVR Lead aVL


www.eeecube.com

www.eeecube.com

www eeecube com


46/56

.

Vout

+

-

Lead aVF

www.eeecube.com

www.eeecube.com

www eeecube com


47/56


The ECG is recorded between a single exploratory electrode and the

central terminal which has a potential corresponding to the center ofthe body

Two equal and large resistors are connected to a pair of limb electrodes

The center of this resistive network acts as central terminal and the

remaining limb electrode acts as the exploratory electrode

A slight increase in the ECG voltage can be realized

The lead connections are

augmented Voltage Right arm (aVR)

augmented Voltage Left arm (aVL)

augmented Voltage Foot (aVF)

www.eeecube.com

www.eeecube.com

1 mVwww eeecube com


48/56

o o

DefibrillatorProtection

Circuit

LeadSelector Bioamp

CM ReductionAmplifier

IsolationTransformer

IsolatedPowerSupply

PowerSupply

OutputUnit

o

o

a.c.Power

Calibration

1 k

1 M1 V

aVRaVL

aVF

COFF

1

2

3

ECG Recording Set Up

www.eeecube.com

www.eeecube.com

www eeecube com


49/56

Electroencephalography

Deals with the recording and study of electrical activity of the brain

By means of electrodes attached to the skull of a patient, the brain

waves are picked up and recorded

Helps to diagnose

Level of consciousness

Sleep disorders

Brain tumors

Epilepsy

Brain death

Multiple sclerosis

www.eeecube.com

www.eeecube.com

www eeecube com


50/56

Stage 1 Stage 2 Stage 3 Stage 4 Stage 5

DriverAmplifier

ControlUnit

Pre Amplifier

Input

Input

Input TraceAmplifier Writer

Block Diagram of EEG Recording Set Up

www.eeecube.com

www.eeecube.com


51/56

www eeecube com
http://www.eeecube.com/http://www.eeecube.com/http://www.eeecube.com/


52/56

.

..Input EMG

Amp.

Oscilloscope

TapeRecorder

A.F.Amp.

Speaker

Block Diagram of EMG Recording Set Up

www.eeecube.com

www.eeecube.com

www.eeecube.com


53/56

The surface or needle electrodes pick up the potentials produced by

the contracting muscle fibers

Types of electrodes

Bipolar two surface electrodes

Unipolar one surface and one needle electrode

The signals are displayed on a cathode ray oscilloscope

The signals are also recorded in the tape recorder for future

reference

The sounds from the loud speaker can be used to diagnose theneuromuscular disorders

www.eeecube.com

www.eeecube.com

www.eeecube.com


54/56

Determination of conduction velocities in motor nerves

The nerve function is examined directly at the various segments of the nerve by means of

stimulating it with a brief electric shock

By measuring the latencies the conduction velocity is calculated

Latency elapsed time between the stimulating impulse and the muscles action potential

Procedure

The EMG electrode and the stimulating electrode are placed at two points on the skin,

separated by a known distance l1

A brief electrical pulse is applied through the stimulating electrode

When the excitation reaches the muscle, this contracts with a short twitch

The elapsed time t1, between the stimulating impulse and muscles action potential, is

measured

Process is repeated for distance l2 < l1, the duration is t2 seconds

The conduction velocity, v = l1 l2

t1 t2

www.eeecube.com

www.eeecube.com

www.eeecube.com


55/56

Electroretinography

Recording and interpreting the electrical activity of eye

All sense organs are connected to the brain but the eye has a special

relationship as the retina is the extension of the cerebral cortex

Potentials within the eye can be easily recorded because of its

exposed position

If the illumination of the retina is changed, the potential changes

slightly in a complex manner

The recording of these changes is called the electroretinogram

When light falls on the retina, the absorption of photons by photo

pigments localized in the outer segment of the retinas

photoreceptors takes placewww.eeecube.com

www.eeecube.com

www.eeecube.com


56/56

This causes the breakdown of photo pigments which results in the

liberation of ions that cause a change in the membrane potential

This in turn results in the development of action potential that is

transmitted down the optic nerve

This action potential is picked up by the electrodes and fed to the

bio amplifier and recorder The recording set up is similar to ECG

eeecube co

Date post:	03-Jun-2018
Category:	Documents
Upload:	karthiha12
View:	218 times
Download:	0 times

bmi Unit 2_opt.pdf

Documents